I often encounter code like this:

if (rowCount > rowIdx)
    {
      if (drc[rowIdx].Table.Columns.Contains("avalId"))
      {
        do
        {
          if (Attributes[attrVal.AttributeClassId] == null)
          {
            // do stuff
          }
          else
          {
            if (!(Attributes[attrVal.AttributeClassId] is ArrayList))
            {
              // do stuff
            }
            else
            {
              if (!isChecking)
              {
                // do stuff
              }
              else
              {
                // do stuff
              }
            }
          }
          rowIdx++;
        }
        while (rowIdx < rowCount && GetIdAsInt32(drc[rowIdx]) == Id);
      }
      else
        rowIdx++;
    }
    return rowIdx;
  }  

The excessive nesting of conditional clauses pushes the code out into an arrow formation:

if if if if do something endif endif endif endif

And you know you're definitely in trouble when the code you're reading is regularly exceeding the right margin on a typical 1280x1024 display. This is the Arrow Anti-Pattern in action.

One of my primary refactoring tasks is "flattening" arrow code like this. Those sharp, pointy barbs are dangerous! Arrow code has a high cyclomatic complexity value – a measure of how many distinct paths there are through code:

Studies show a correlation between a program's cyclomatic complexity and its error frequency. A low cyclomatic complexity contributes to a program's understandability and indicates it is amenable to modification at lower risk than a more complex program. A module's cyclomatic complexity is also a strong indicator of its testability.

Where appropriate, I flatten that arrow code by doing the following:

1. Replace conditions with guard clauses. This code..

   if (SomeNecessaryCondition)
   {
     // function body code
   }
   

   .. works better as a guard clause:

   if (!SomeNecessaryCondition)
   {
     throw new RequiredConditionMissingException;
   }
   // function body code
   

2. Decompose conditional blocks into seperate functions. In the above example, we're in a do..while loop which could be decomposed:

   do
   {
     ValidateRowAttribute(drc[rowIdx]);
     rowIdx++;
   }
   while(rowIdx < rowCount && GetIdAsInt32(drc[rowIdx]) == Id);
   

3. Convert negative checks into positive checks. As a broad rule, I prefer to put the positive comparison first and let the negative comparison fall out naturally into the else clause. I think this reads a lot better and, more importantly, avoids the "I ain't not never doing that" syndrome:

   if (Attributes[attrVal.AttributeClassId] is ArrayList)
   {
     // do stuff
   }
   else
   {
     // do stuff
   }
   

4. Always opportunistically return as soon as possible from the function. Once your work is done, get the heck out of there! This isn't always possible – you might have resources you need to clean up. But whatever you do, you have to abandon the ill-conceived idea that there should only be one exit point at the bottom of the function.

The goal is to have code that scrolls vertically a lot… but not so much horizontally.